918.4 Circulating air ducts
For fuel-burning, forced-air warm-air furnaces, circulating air must be conducted into the blower housing from outside the furnace enclosure through continuous airtight ducts. The requirement addresses duct continuity and airtightness. It does not itself require 100 percent outdoor air. This subsection specifically addresses fuel-burning furnaces.
918.5 Outdoor and return air openings
Two separate location checks apply:
Outdoor intake openings: IMC 401.4
Return-air openings: IMC 601.5
Section 918.5 identifies the governing references. Verify the actual location criteria in those sections rather than assuming one rule covers both openings.
918.6 Outdoor opening protection
Outdoor air intake openings must be protected in accordance with IMC 401.5. Location and protection are separate checks. A compliant intake location does not, by itself, establish compliant opening protection.
Where to show it
Mechanical plans: circulating-air route and opening locations
Furnace section or detail: continuous airtight duct connection into the blower housing
Exterior opening detail: protection coordinated with Section 401.5
Architectural plans: room uses and openings needed for location review
Do
Trace the circulating-air path all the way to the blower housing.
Check outdoor intake and return-air locations separately.
Coordinate intake protection with the referenced section.
Do not
Confuse air from outside the furnace enclosure with a requirement for outdoor air.
Treat opening location and protection as the same requirement.
Field tip
Follow the return-air path on both plan and section. At the furnace enclosure, confirm that the detail actually shows the continuous airtight duct connection, not just an arrow pointing toward the equipment. Next: IMC 918 Forced-Air Warm-Air Furnaces Recap.
In Florida
In Florida the mechanical code is the 2023 FBC-Mechanical (8th Edition), which is based on the 2021 IMC and generally follows the same Chapter 9 numbering. Confirm the Florida section text, any Florida amendment and the referenced standard editions before relying on a 2024 IMC detail. The 9th Edition applies to permit applications on or after 12/31/2026 (state adoption pending).
Questions to settle before design
Where does the circulating air come from, and how does the duct connect into the blower housing?
Where are the outdoor intake and the return openings relative to each other and to contaminant sources?
What protects the outdoor intake (screen, louver or grille), and is it shown on a detail?
MasterBuild field checklist
Inspect the duct-to-furnace connection for continuity and sealing, not just presence.
Confirm the installed intake protection matches the detail.
Follow the return path on site from the grille back to the furnace.
MasterBuild recommendation: process steps for design and site review, not code text.
Where MasterBuild helps
PE-led MEP design in Miami, or send your project background. The fee is quoted in writing before work starts.
Code basis
2024 IMC Sections 918.4-918.6 model-code education from the owner-published Daily Code Talk. Confirm the locally adopted edition and amendments, the selected product listing and manufacturer instructions, and the authority having jurisdiction. Read the ICC 2024 IMC Chapter 9 for the complete model-code provisions.